1. Technical Field
This invention relates generally to phase shifting circuits and in particular to a variable phase amplifier circuit and its use in tuning devices having resonators.
2. State of the Art
Some electromechanical oscillators include a mechanical resonator coupled to an electronic driving circuit. The electronic driving circuit produces a drive signal to drive the mechanical resonator. The mechanical resonator has an inherent resonance frequency. The inherent resonance frequency of the mechanical resonator is determined, at least partially, by factors that are subject to design choice such as size, shape, and resonator material. The mechanical resonator may be designed to have a desired resonance frequency by suitably designing those factors. However, due to manufacturing tolerances resulting in deviations of the design factors from their intended values, and due to variations arising during operation of the electromechanical oscillator (for example, temperature induced variations, ambient pressure variations, package-induced stress, material dependent stress), the mechanical resonator may not have the desired inherent resonance frequency in all situations.
To account for manufacturing tolerances and temperature induced shifts in the resonance frequency of the mechanical resonator, the electromechanical oscillator may be tuned. FIG. 1 illustrates a characteristic impedance curve of a resonator as a function of frequency, which curve is applicable to the mechanical resonators of typical electromechanical oscillators. The impedance is illustrated by line 102, and includes both a series resonance peak 104, occurring at a resonance frequency fres, and a parallel resonance peak 106. In the case of electromechanical oscillators having a mechanical resonator, conventional tuning methods allow for tuning of the oscillator operation within a tuning range 108 between the series resonance peak and the parallel resonance peak. Because the conventional tuning range 108 occurs on only one side of the series resonance frequency, fres, the accuracy of tuning of the oscillator around the series resonance frequency of the mechanical resonator is limited. Accordingly, it is desirable to have a device which is capable of tuning a mechanical resonator in a tuning range that extends above and below the series resonance peak of the mechanical resonator.